Electrostatic hold down apparatus

ABSTRACT

Electrostatic hold down apparatus for effecting transfer of a tacky toner image from the photoconductive surface of a drum to a sheet of paper or the like for which the toner has greater affinity than for the drum surface in which the paper is brought to the same potential as a plate extending over an area of and in closely spaced relationship to the drum surface as the paper enters the space between the plate and the drum with the plate being placed at a relatively high negative potential with respect to the drum to cause electrostatic forces to bring the portion of the paper beneath the plate into intimate contact with the drum surface. As the paper leaves the space between the drum and the plate the electrostatic hold down force is relieved so that the paper is easily stripped from the drum and the image, owing to the greater affinity of the tacky toner for the paper, leaves the drum with the paper.

llnite Van Vloten tea tet 1 1 1 Dec. 3, 1974 1 1 ELECTROSTATIC HOLD DOWN [73] Assignee: Savin Business Machines Corporation, Valhalla, NY.

22 Filed: Aug. 29, 1973 211 Appl. No.: 392,746

[52] US. Cl. 355/3 R, 271/DIG.3, 226/94, 96/1.4, 250/324 [51] Int. Cl G03g 15/16 [58] Field of Search 355/3 R; 271/D1G. 3; 226/94; 96/14; 117/175; 250/324, 325, 326

[56] References Cited UNITED STATES PATENTS 3,146,385 8/1964 Carlson 250/325 3,240,933 3/1966 Reinke 250/324 3,405,052 10/1968 Schirmerm 250/324 X 3,578,859 5/1971 Stillings 355/3 R 3,620,617 11/1971 Kelly et a1 i 355/3 R 3,649,262 3/1972 Cade et al 355/15 X HIGH VOLTAGE POW ER SUPPLY Primary ExaminerRobert P. Greiner Attorney, Agent, or Firm-Shenier & OConnor [57] ABSTRACT Electrostatic hold down apparatus for effecting transfer of a tacky toner image from the photoconductivc surface of a drum to a sheet of paper or the like for which the toner has greater affinity than for the drum surface in which the paper is brought to the same potential as a plate extending over an area of and in closely spaced relationship to the drum surface as the paper enters the space between the plate and the drum with the plate being placed at a relatively high negative potential with respect to the drum to cause electrostatic forces to bring the portion of the paper beneath the plate into intimate contact with the drum surface. As the paper leaves the space between the drum and the plate the electrostatic hold down force is relieved so that the paper is easily stripped from the drum and the image, owing to the greater affinity of the tacky toner for the paper, leaves the drum with the paper.

4 Claims, 2 Drawing Figures PATENTELDEB 31914 HIGH VOLTAGE POWER SUPPLY HIGH VOTAGE POWER su pw ELECTROSTATIC HOLD DOWN APPARATUS BACKGROUND OF THE INVENTION Conventional xerographic copiers of the image transfer type utilize electrostatic forces for both of the principal phases of the photocopy process which are image formation on the photoconductive surface of the drum and image transfer to a final image-bearing sheet material, such as paper or the like. In the first phase, a corona discharge is applied to the photoconductive surface of the drum to produce a uniformly distributed electrostatic charge thereover. Light reflected from the specimen to be copied is projected onto the surface of the drum to render it conductive in non image areas to permit charge to leak off in the area of exposure and thus to leave a latent electrostatic image of the specimen upon the drums surface. The image is developed by rotating the drum through a suspension of toner and carrier particles wherein the frictional interactionbetween these two developer components has caused the toner particles to acquire charges opposite to those on the drums surface. The oppositely charged toner particles adhere to the drum in those areas defined by the latent electrostatic image while the. like-charged carrier particles remain in the developer suspension.

In the second phase of the process the developed image is transferred from the drums surface to the sheet material. A secondcorona discharge is used to charge the latter which is brought into close proximity with the former whereupon the charged toner image is electrostatically transferred to the surface of the sheet material. The final image is made permanent by a critically controlled heating of the sheet material to a temperature sufficient to fix the image without burning or discoloring the sheet material.

Copending application 155,108 filed by Smith et al. on June 21, 1971, discloses an improved system for image transfer from drum to sheet material utilizing'a tacky toner'whi'ch has a greater affinity for the sheet material than for the drums photoconductive surface. Unlike the electrostatic transfer process, mere application of the sheet material to the'drums surface under pressure effects image transfer. The pressure necessary to achieve image transfer in Smith et al. is applied by a roller which presses the sheet material against the drum. While the system successfully achieves image transfer without the use of an auxiliary image transfer system it incorporates a number of defects. The roller has limited area contact with the drum producing nearly line contact which may allow slippage between paper and drum. Furthermore, a misfed piece of paper or operation of the machine without paper may cause direct contact between roller and drum necessitating cleaning of the roller and possibly damaging the drum. My invention avoids these disadvantages.

SUMMARY OF THE INVENTION static hold down apparatus which produces simultaneous drum-to-paper contact over a broad area thereby enhancing image quality and preventing slippage between drum and paper.

Other and further objects of my invention will appear 7 from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS conjunction therewith and in which like reference characters indicate like parts in the various views:

FIG. 1 is a perspective view of my electrostatic hold down apparatus" FIG. 2 is a side elevation of the electrostatic hold down apparatus shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, the surface of a drum 10, forming part of an electrostatic copier, and adapted to be driven by any suitable means (not shown) carries a photoconductive coating 12 adapted to receive a latent electrostatic image which is developed, for example, with a developer comprising tacky toner particles suspended in a diluent in the manner described in the Smith et al. application, supra. My hold down apparatus includes an arcuate conductive plate 14 mounted parallel to and in close proximity to the surface of drum 12 to form a paper passage 16 between drum 10 and plate 14. A lip 18 extending along the lower edge of plate 14 forms a mouth 20 opening into passage 16.

Any suitable means such as screws 22 or the like secure a wide conductive brush 24 to, and in electrical contact with, the lip 18 so that the brush extends across mouth 20 toward the surface 12 of the drum 10. An elongated wedge-shaped guide 26 positioned below mouth 20 directs the leading edgeof a length of paper 28 into engagement with brush 24 as it enters mouth 20.

A brush 30 in engagement with drum l0 maintains the drum at ground potential. My apparatus includes a power supply 32 of any suitable type known to the art which produces an output potential of minus 2 to 3 Kv. I connect the negative output terminal to plate 14 to place the plate and brush 24 at a potential of the order of minus '2-3 Kv with respect to the grounded drum l0. 1 may provide my apparatus with a guide 34 for assisting in stripping the paper 28 from the drum 10.

In operation of my apparatus paper 16, on which a copy is to be reproduced, is fed into the mouth 20 and under the brush 24 by any suitable means such as that shown in the Smith et al. application. Upon contact with the brush 24 the paper 28 becomes highly negatively charged. As the charged paper continues along passage 16 the electrostatic field existing therein forces the negatively charged paper 28 in a radial direction toward the axis of the drum 10 on the surface of which a toner image has been formed in the manner described in the Smith et all. application. The paper is repelled by the like negative charges on the plate 14 and attracted by the induced positive charges on the photoconductive surface 12. The hold down force is computed by F Eq where E is the intensity of the electrostatic field between the plate 14 and drum l0 and q is the charge on the paper 28. Sufficient force is generated to cause transfer of the tacky toner image from the drums surface to the paper unassisted by any external mechanical means such as pressure rollers. In an experimental set up with a plate at minus 3 Kv a hold down force of approximately 14 oz/in was produced. Once the paper exits from the electrostatic field in the transfer area, the

.hold down force is substantially reduced thereby facilitating removal of the paper from the drum by guide 34, for example.

Rather than using an elongated brush 24 I might employ a plurality of wire brushes, for example. In addition, means other than a drum may be used for forming a toner image and applying it to a contacting sheet of paper, such as a belt-like apparatus.

It will be seen that l have accomplished the objects of my invention. My apparatus produces intimate contact between paper and drum with force sufficient to transfer a toner image without the use of mechanical means such as pressure rollers. In addition, contact is achieved over a broad area of the drum and paper surfaces simultaneously.

It will be understood that certain features and subcombinations'are of-utility and may be employed with- Having thus described my invention, what I claim is:

1.- In a copier having a conductive support the surface of which carries photoconductive material adapted to receive an electrostatic image which is developed with tacky toner and which is to be transferred to a sheet of copy material fed to said surface at a transfer station,

apparatus for electrostatically holding said sheet of copy material in contact with said surface at said transfer station including a plate-like electrode of conductive material, means mounting said electrode at said transfer station in closely spaced relationship to said surface to form a passage having an entrance into which said sheet is introduced and-an exit from which said sheet emerges in the course of a transfer operation, a source of potential, means for applying said potential to said electrode to produce a static electrical field in said passage, and means for applying said potential directly to said sheet as it moves into said entrance.

2. Apparatus as in claim 1 in which said plate-like electrode extends over an appreciable area of said surface.

3. Apparatus as in claim 2 in which said means for applying said potential to said sheet is a conductive brush carried by said electrode and adapted to contact said sheet as'it enters said entrance.

4. Apparatus as in claim 3 in which said support is a section. 

1. In a copier having a conductive support the surface of which carries photoconductive material adapted to receive an electrostatic image which is developed with tacky toner and which is to be transferred to a sheet of copy material fed to said surface at a transfer station, apparatus for electrostatically holding said sheet of copy material in contact with said surface at said transfer station including a plate-like electrode of conductive material, means mounting said electrode at said transfer station in closely spaced relationship to said surface to form a passage having an entrance into which said sheet is introduced and an exit from which said sheet emerges in the course of a transfer operation, a source of potential, means for applying said potential to said electrode to produce a static electrical field in said passage, and means for applying said potential directly to said sheet as it moves into said entrance.
 2. Apparatus as in claim 1 in which said plate-like electrode extends over an appreciable area of said surface.
 3. Apparatus as in claim 2 in which said means for applying said potential to said sheet is a conductive brush carried by said electrode and adapted to contact said sheet as it enters said entrance.
 4. Apparatus as in claim 3 in which said support is a drum and in which said electrode is arcuate in cross-section. 